The invention relates to a detector device that is particularly useful for measuring EUV in-band intensity in photolithography systems. The detector can be readily adapted as a transferable standard from synchroton systems to laser-produced plasma radiation sources.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a cast image of the subject pattern. Once the image is cast, it is indelibly formed in the coating. The recorded image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent or opaque to the impinging radiation. Exposure of the coating through the transparency placed in close longitudinal proximity to the coating causes the exposed area of the coating to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing as described in the previous paragraph. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of 10 to 20 nm) are now at the forefront of research in efforts to achieve smaller transferred feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection (demagnifying) lens onto a wafer. Reticles for EUV projection lithography typically comprise a glass substrate coated with an EUV reflective material and an optical pattern fabricated from an EUV absorbing material covering portions of the reflective surface. In operation, EUV radiation from the illumination system (condenser) is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation is re-imaged to the wafer using a reflective optical system and the pattern from the reticle is effectively transcribed to the wafer.
A source of EUV radiation is the laser-produced plasma EUV source, which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit EUV radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line so that malfunction does not close down the entire plant. A stepper employing a laser-produced plasma source is relatively inexpensive and could be housed in existing facilities. It is expected that EUV sources suitable for photolithography that provide bright, incoherent EUV radiation and that employ physics quite different from that of the laser-produced plasma source will be developed. One such source under development is the EUV discharge source.
EUV lithography machines for producing integrated circuit components are described for example in U.S. Pat. No. 6,031,598 to Tichenor et al. Referring to FIG. 4, the EUV lithography machine comprises a main vacuum or projection chamber 2 and a source vacuum chamber 4. Source chamber 4 is connected to main chamber 2 through an airlock valve (not shown) which permits either chamber to be accessed without venting or contaminating the environment of the other chamber. Typically, a laser beam 30 is directed by turning mirror 32 into the source chamber 4. A high density gas, such as xenon, is injected into the plasma generator 36 through gas supply 34 and the interaction of the laser beam 30, and gas supply 34 creates a plasma giving off the illumination used in EUV lithography. The EUV radiation is collected by segmented collector 38, that collects about 30% of the available EUV light, and the radiation 40 is directed toward the pupil optics 42. The pupil optics consists of long narrow mirrors arranged to focus the rays from the collector at grazing angles onto an imaging mirror 43 that redirects the illumination beam through filter/window 44. Filter 44 passes only the desired EUV wavelengths and excludes scattered laser beam light in chamber 4. The illumination beam 45 is then reflected from the relay optics 46, another grazing angle mirror, and then illuminates the pattern on the reticle 48. Mirrors 38, 42, 43, and 46 together comprise the complete illumination system or condenser. The reflected pattern from the reticle 48 then passes through the projection optics 50 which reduces the image size to that desired for printing on the wafer. After exiting the projection optics 50, the beam passes through vacuum window 52. The beam then prints its pattern on wafer 54.
Although no longer under serious consideration for high-volume commercial fabrication applications, synchrotron sources play an extremely important role in the development of EUV lithography technology. Being readily available, highly reliable, and efficient producers of EUV radiation, synchrotron radiation sources are well suited to the development of EUV lithography. These sources are currently used for a variety of at-wavelength EUV metrologies such as reflectometry, interferometry, and scatterometry.
As is apparent, lithography systems include a number of vacuum compartments through which EUV radiation is processed. Measuring and regulating the EUV radiation intensity through the lithography system is critical to maximizing performance. Prior art techniques for measuring the flux typically employed devices with EUV-sensitive vacuum photodiodes, which were difficult to calibrate. The art is in need of a reliable, cost effective EUV radiation intensity detector that is able to survive large pressure impulses without affecting calibration. The detector should also have a compact, noise immune design.
The present invention is based in part on the development of a rugged universal in-band detector for detecting extreme ultraviolet radiation that includes:
(a) an EUV sensitive photodiode having a diode active area that generates a current responsive only to EUV radiation;
(b) one or more mirrors that reflects EUV radiation having a defined wavelength(s) to the diode active area; and
(c) a mask defining a pinhole that is positioned above the diode active area, wherein EUV radiation, generated on a remote target, is imaged and aperture through the pinhole.
In a preferred embodiment, the detector comprises a housing assembly that has:
(i) a chamber in which the EUV sensitive photodiode is positioned; and
(ii) an entrance through which EUV radiation enters the chamber.
The robust universal in-band detector has a number of advantages over conventional EUV detectors including higher intrinsic calibration accuracy, precision, and mechanic robustness in a vacuum environment.